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Date: May 27, 2010

Listed below is further information about methods used by the U.S. Inventory of 

Greenhouse Gas Emissions and Sinks as well as Natural Gas STAR presentations and 

documents that cite inventory information.

 Question: Can you elaborate on the reference for methane emissions pie charts 

used in Natural Gas STAR Presentations?

o Response: Consult the Energy section (Section 3) and Sections 3.4 and 

3.5 of the Annexes in the 2007 U.S. Inventory found at 

epa.gov/climatechange/emissions/usgginv_archive.html. The individual 

source categories and their methane emissions begin on page A-143 for 

natural gas and page A-151 for petroleum in Annex 3. The Annex more 

thoroughly covers the methodology used to arrive at the results in Section 

3.

 Question: Why are REC reductions greater than the entire inventory number for 

well completions?

o Response: Presently, REC reductions reported in the Natural Gas STAR 

body of work is larger than well completion venting in the inventory on an 

annual basis. Section 3 of the inventory discusses improvements and 

future plans for each sector. For the natural gas industry, the planned 

improvements are discussed on pg. 3-46. For the petroleum industry, the 

planned improvements are discussed on pg. 3-51.

In a New York Times article published on October 15, 2009, it is 

mentioned that the EPA is currently reviewing and revising methane 

emissions from U.S. gas wells. As a result, the EPA found that the 

amount emitted by routine operations at gas wells is approximately 12 

times larger than the existing estimate of 9 Bcf. The NYT article can be 

found here at www.nytimes.com/2009/10/15/business/energyenvironment/15degrees.html#.

EPA has included these revised emission numbers in the upcoming 

Mandatory Reporting Rule (MRR). Specifically, these new numbers are 

in the Technical Support Document (TSD), explained in Appendix B (pgs. 

79-85). On page 82, the new 2007 U.S. emissions from well completion 

and workover venting is approximately 120 Bcf. The TSD can be found 

here at www.epa.gov/climatechange/emissions/downloads10/SubpartW_TSD-022310.pdf.

 Question: Why are Natural Gas STAR methane emissions reductions accounted 

for entirely in the natural gas systems sector of the inventory and not at all in the 

petroleum systems sector?
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o Response: The methodology in Annex 3 of the inventory indicates how 

Natural Gas STAR reductions are applied to the natural gas systems 

sector.

Natural Gas STAR considers data at the level of the production sector, 

rather than further dividing its reporting into natural gas and petroleum. 

At the same time, the inventory considers data at the level of natural gas 

systems production & petroleum systems production. Often a Natural Gas 

STAR reduction activity straddles both natural gas systems and petroleum 

systems and is therefore inherently an overall production sector activity. 

The Natural Gas STAR production sector reductions are not further 

deconstructed and are thus applied to natural gas systems production 

sector. For example, vapor recovery units (VRUs) can be installed on 

both condensate (gas) and storage (oil) tanks, but it is not a Natural Gas 

STAR reporting requirement to specify which.

 Question: How can natural gas and petroleum systems methane emissions be 

expressed as a percentage of total gas throughput entering and exiting the national 

supply chain?

o Response: This percentage is not a published statistic but can be roughly 

calculated from public sources such as the inventory and such as EIA gas 

production data. It is also important to understand numerical treatment of

gas imports and exports, NGL imports and exports, gas reinjected, gas 

consumed on site, gas unaccounted for due to intrinsic metering errors, 

and distinguishing between methane versus natural gas data, which all 

have first order effects on the resulting percentage. Any result using this 

calculation method would not apply to any specific site given the topdown nature of the calculation.

To estimate the percentage of gas lost from both the oil and gas industry, 

two public sources are used: 1) information from the Energy Information 

Administration (EIA) and 2) the Technical Support Document (TSD) for 

the MRR. The equation shown below illustrates how the percentage can 

be calculated.

Where:

%gaslost = Estimated percentage lost from 

produced gas due to methane 

emissions

ETotal,2006 = 2006 revised methane emissions for 

petroleum and gas, all sectors (with 

reductions), from TSD which 

,2006 Pr ,2006

,2006 %

Gross oduction

Total

gaslost W E

E




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includes revised estimation methods 

for well completion emissions

WGross,2006 = EIA 2006 gross gas withdrawals

EProduction,2006 = 2006 revised methane emissions for 

petroleum and gas production (with 

reductions) from TSD which 

includes revised estimation methods 

for well completion emissions

The term EProduction,2006 in the denominator of the above equation was 

included because the gross withdrawals, WGross,2006 does not account for 

the original amount of gas produced from a given well. This is because 

WGross,2006 is based on meter readings which typically occur as the gas 

exits the production site, after it has encountered all production equipment 

that contribute to method emissions. Therefore, the emissions from the 

production sector must be added back in to truly have the entire amount of 

gas withdrawn from wells.

Both revised TSD terms in the equation include Natural Gas STAR 

reductions and were calculated from consulting page 9 of the TSD and 

subtracting out the 2006 reductions listed in Table A-123 on page A-151 

of the 2008 U.S. Inventory Annex 3. A summary table is shown below 

and gives the value of each equation term along with the calculated 

percentage of gas lost in the petroleum and gas industry due to methane 

emissions.

Determination of Gas Lost Due to Methane Emissions in the Oil and Gas Industries

Equation Term Calculation of Term MMtCO2e

ETotal,2006

257.2 MMtCO2e from gas1+ 26.3 

MMtCO2e from oil1– 45.8 MMtCO2e 

NGS reductions2

237.7

WGross,2006

23,535,018 MMcf3converted to 9519.0 

MMtCO2e 9519.0

EProduction,2006

0.66(257.2 MMtCO2e)4+ 0.97(26.3 

MMtCO2e)4– 30.8 MMtCO2e NGS 

production reductions5

164.5

Percentage of gas lost 2.45%

1MRR. Technical Support Document. page 9. 

www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD-022310.pdf

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EPA. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008. Annex 3: Section 3.4. Table A123: CH4 reductions derived from the Natural Gas STAR Program 

http://www.epa.gov/climatechange/emissions/usinventoryreport.html

2,182 Gg x 109g/Gg x 21g CO2/g CH4 x Mg/106g x MMtCO2e/106Mg = 45.8 MMtCO2e

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EIA website: www.eia.doe.gov/dnav/ng/ng_prod_sum_dcu_NUS_a.htm

23,535,018 MMcf x 106cf/MMcf x 19.26g/cf x 21g CO2/g CH4 x Mg/106g x MMtCO2e/106Mg = 

9519.0 MMtCO2e

4 MRR. Technical Support Document. page 9. 

www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD-022310.pdf

-Gas production emissions account for 66% of total methane emissions from gas industry = 169.8 

MMtCO2e
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-Oil production emissions account for 97% of total methane emissions from oil industry = 25.5 

MMtCO2e

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EPA. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008. Annex 3: Section 3.4. Table A123: CH4 reductions derived from the Natural Gas STAR Program 

http://www.epa.gov/climatechange/emissions/usinventoryreport.html

1,469 Gg x 109g/Gg x 21g CO2/g CH4 x Mg/106g x MMtCO2e/106Mg = 30.8 MMtCO2e

Notes:

MMtCO2e = million metric tonnes carbon dioxide equivalent

MMcf = million cubic feet

Gg = gigagrams

Mg = megagrams

 Question: Are changes/updates to the national inventory tracked every year? 

Why would previous year results change?

o Response: Yes, for each annual inventory cycle, the entire time series is 

considered and updated. The entire time series of 1990 to the present 

reporting year is updated annually because revised activity data is applied 

as it is available. It is not uncommon for EIA or other data sources to 

issue revised estimates for years in the recent past, and such revisions are 

tracked by the inventory. Any changes in the data sources used are be 

noted in the executive summary and introduction sections of the inventory.